Salient and smooth edge ramps inducing turbulent boundary layer separation: flow characterization for control perspective A. Debien, S. Aubrun, N. Mazellier, A. Kourta Projet ANR- 11-BS09-018 SePaCoDe
Introduction Turbulent boundary layer (TBL) separation Many industrial processes (airfoils, turbomachines, diffuser ) Detrimental effect (drag increase, lift reduction, pressure loss...) Flow control to achieve performance enhancement Active flow control strategy Transposition to full-scale application remains of limited success 2
ANR SePaCoDe Separation Control-from passive to closed-loop Design LML, PPRIME, PRISME Improve physical insight into the control process of separating wallbounding shear-flows Ramp configuration 2D geometry Salient edge (geometrical singularity) Smooth edge (adverse pressure gradient) Laminar and TBL separation on analogue configurations Clarify flow physics cause/effect Develop a hierarchy of robust control strategies More reliable transposition to full-scale applications 3
PRISME laboratory experiments TBL separation on salient and smooth edges ramp (2 m span) Re θ O 10 3 δ θ O 10 3 m Shedding frequency F shed O 10 2 Hz Control by Active Vortex Generators (VGA) Configuration based on Cuvier et al., TSFP7 2011 Adapted to PRISME configuration ramp Presentation purpose : Baseline flow to prepare flow control experiments 4
Active Vortex Generators Counter-rotating VGA (Cuvier et al., TSFP7 2011) Skew angle β = 45 Pitch angle α = 135 Jet diameter ϕ δ = 0.03 0.06 λ ϕ = 30 L ϕ = 15 Distance VGA - x sep : Δx VG δ = 1 Velocity ratio V jet U ref = 3 Frequency : related to baseline time-scale Duty cycle = 50% Counter-rotating jet parameters, Cuvier et al., TSFP7 2011 Confirmed / determined by salient and smooth edge baseline experiments 5
Experimental setup - test section (Malavard, PRISME): 2 X 2 m² U 60 m/s Turbulence level 0.4 % - salient and smooth edge ramps h = 100 mm l = 470 mm Slant angle of 25 2 m span - U 0 = 20 m/s, Re θ = 0(10 3 ) - tripped Boundary layer 300-µm thickness zig-zag 103 mm downstream from leading edge 6
Experimental measurements 112 pressure tranducers (PSI) Cp and gradient Cp 7 Single hot-wire (Dantec 55P11) Upstream properties of TBL Cross hot-wire (Dantec 55P61) Time-scale properties of shear layer PIV (TSI) Flow field over the ramp 2 x JAI 12-bit cameras, pixel size 190 µm 7
Upstream flow properties 8
Upstream flow Pressure coefficient distribution Pressure coefficient vanishes salient edge : -8.7 < x/h < -4.7, dc p dx < 0.044 m-1 smooth edge : -8.7 < x/h < -5.7, dc p dx < 0.035 m-1 Pressure gradient distribution Smooth edge, x/h > -2 Strong favourable pressure gradient induced by flow expansion Existence of low pressure gradient zone upstream from ramp 9
Upstream flow Salient edge ramp Salient edge Smooth edge x/h 1.27 0 U e (m/s) 21.77 21.25 δ 99 (mm) ~ 22 ~ 22 δ* (mm) 3.35 3.08 δ θ (mm) 2.6 2.37 Re θ 3628 3223 H 12 1.28 1.3 u τ (m/s) 0.90 0.833 τ w (kg/m.s 2 ) 0.96 0.826 Smooth edge ramp 10
VGA configuration 11
Separation zone Mean streamwise velocity for salient and smooth edge ramp Salient edge ramp separation point x sep /h = 1.27 reattachment point x/h = 6.57 Smooth edge ramp separation point x sep /h = 2.17 reattachment point x/h = 4.86 At mean separation point δ 99 = 0.46 h 12
Separation point smooth edge ramp Unsteady position of separation point Intermittence of smooth edge ramp separation Induced by adverse pressure gradient separation Standard deviation σ sep = 0.24h Incipient detachment for x/h > 1.8 Transitory detachment at x/h = 2.24 Mean separation point at x/h = 2.17 To be efficient, VGA must be set upstream from incipient detachment Simpson et al., JFM 1989 13
VGA Configuration Distance VGA - x sep : Δx VG δ = 1 x VG upstream of incipient detachment (x ID ) Jet diameter φ δ = 0.03 0.06 ϕ = 1.2 mm Salient edge Smooth edge x sep /h 1.27 2.27 x ID /h - 1.8 δ 99 /h 0.22 0.46 x VG /h 1.07 1.65 φ δ 0.055 0.025 14
Time-scale properties 15
Time scale properties of shear layer PSD of transverse velocity Shear layer instability progressively reduces Salient edge : from 110 Hz (St θ = 0.014, above x sep ) to 20-25 Hz (St Lsep = 0.53 0.66, downstream from recirculation) Smooth edge : from 70 Hz (St θ = 0.010, above x sep ) to 30-50 Hz (St Lsep = 0.4 0.67, downstream from recirculation) salient edge smooth edge For smooth edge ramp, existence of low frequency mode 16
Separation zone Turbulent kinetic energy for salient and smooth edge ramp Maximum levels of tke in shear layer TKE increase progressively from separation point to reattachment point For smooth edge configuration, TKE decrease beyond reattachment point 17
Separation zone Turbulent kinetic energy for salient and smooth edge ramp Pressure coefficient distribution Maximum levels of tke in shear layer TKE increase progressively from separation point to reattachment point For smooth edge configuration, TKE decrease beyond reattachment point => shear layer deflects toward downstream flat plate can be at the root of low frequency mode 18
Conclusion Upstream flow properties TBL developed upstream from ramp under a low gradient pressure Above separation point, TBL presents Re θ O 10 3 and δ θ O 10 3 m VGA configuration Confirmation/determination of VGA location for both configurations For adverse pressure gradient (APG) separation, VGA needs to be located upstream from incipient detachment Time scale properties Identification of shear layer instability which progressively reduces up to time scale T = O 10 1 10 2 s downstream of reattachment point APG separation presents a low frequency mode 19
Perspectives Baseline post-processing Identification of low frequency mode root (shear layer fluctuation?) Separation of low frequency mode in cross hot-wire data Active flow control of configurations Analyse VGA s pulsation frequency effect Determine time response of separation/ reattachment process 20
Thanks for your attention 21